This invention relates to the production of ferric sulfate, and in particular to the production of ferric sulfate and hydrochloric acid from ferrous chloride or liquors containing ferrous chloride produced in the production of steel or from other industries.
In the processing of steel, steel is pickled with hydrochloric acid. The pickling of the steel produces a waste liquor containing FeCl.sub.2 and HCl. Often, the liquor is discarded, adding to the already existing pollution problems. Various methods have been introduced to treat the pickling liquor. As described in U.S. Pat. Nos. 4,382,916 and 4,222,997, both to Beecher, the hydrochloric acid has been recovered from pickle liquor. The Beecher process produces ferrous sulfate as a by-product. Beecher recovers the hydrochloric acid by condensing pickling liquor to remove HCl and water from the feed stream. He does this by boiling the feed stream. The concentrated pickle liquor is then added to sulfuric acid to produce ferrous sulfate. The HCl and water vapors are recovered using extensive heat recovery equipment.
A roasting process, described in Barczak U.S. Pat. No. 4,436,681 et al., recovers HCl from pickle liquor by injecting the pickle liquor into a roasting chamber at very high temperatures (1,600.degree. F.) using a very high pressure to obtain a fine spray. This process converts the ferrous chloride to Fe.sub.2 O.sub.3 and HCl. It uses a great deal of energy and requires high maintenance costs. The Fe.sub.2 O.sub.3 produced is of poor quality and low value.
These products (ferric oxide, ferrous sulfate, and HCl) are of relatively low value. The processes, which are expensive to setup and operate, therefore, can not be economically justified. However, the ferrous sulfate can be oxidized to produce ferric sulfate. Ferric sulfate has many uses and has a higher value. For example, its main use is as a flocculent to remove particulate matter in potable water plants. It is also used to remove phosphates and H.sub.2 S from waste streams or as an animal food additive. I presently know of no processes that propose to use the ferrous chloride from the pickling liquor to produce ferric sulfate. Such a process is of particular value at this time because of the need to replace alum as a flocculent in the potable water industry. Alum is becoming subject to more restrictive regulations because of its potential dangers. Ferric sulfate is equal to or better than alum as a flocculent without the potential dangers posed by the use of alum.